To protect a steel water heater tank against corrosion it is common to coat the internal surface of the tank with a corrosion resistent coating, such as glass. Although coatings of this type are very effective in protecting the steel tank, intensified corrosion can occur in areas of the tank that are exposed through defects in the glass coating or in areas that are inadequately coated with glass, such as where fittings are connected to the tank. To overcome this problem, sacrificial anode rods are normally mounted within the tank and the anode is composed of a metal, such as magnesium, aluminum or zinc, having a higher electrical potential than the steel tank, with the result that the anode will be preferentially dissipated to protect the steel water heater tank.
In the conventional cathodic protection system, the anode is normally formed with a steel supporting core wire and the upper end of the core wire is connected to a steel cap which in turn is threaded within the internal threaded opening of a steel spud that is welded to the outer surface of the tank head. With this construction, the anode will be suspended from the spud and extends downwardly through an opening in the tank head. In order to provide proper alignment of the anode, the internal threaded opening in the spud has a smaller diameter than the opening in the tank head with the result that there is a space between the anode and the edge of the head bordering the opening.
In the normal production procedure, the anode is assembled to the tank after the tank has been glass coated and fired. During the glass coating operation, it is often difficult to adequately glass coat the edge of the upper head tank that borders the anode opening, and similarly it is difficult to adequately coat the inner edge of the spud which is exposed through the opening. Therefore, during operation of the water heater there may be concentrated consumption of the upper end of the anode adjacent these exposed steel areas. This results in a condition called "necking", in which the upper end of the anode will be necked down or decreased in diameter relative to the remaining portion of the anode. If the "necking" continues, the steel core wire will be exposed at the upper end of the anode which will further increase the consumption of the anode in this area.
In an attempt to overcome the "necking" problem, it is has been proposed to enclose the upper end of the anode in an insulating protective sleeve. However, the use of a protective sleeve has not proven satisfactory because it limits the electrolytic current distribution in the upper end of the tank and thus does not adequately protect exposed areas of steel at the upper head.